Low second grid voltage electron gun



March 1968 TAKAHARU MARUYAMA ETAL 3, LOW SECOND GRID VOLTAGE ELECTRON GUN Filed May 27, 1964 2 Sheets-$heet 1 Fig 1 PRIOR ART INVENTORS Eka'ha/w Marx/y ama fl/ra fum/ Suzuki Macaak/ yd/fldl/C'h/ ATTORNEY March 1968 TAKAHARU MARUYAMA ETAL 3,374,379

LOW SECOND GRID VOLTAGE ELECTRON GUN Filed May 27, 1964 2 Sheets-Sheet 2 PRIOR ART LE -A PEA-B 0 V [1C50V a DCIZOOOV 0v ucsov D.C1?000V m I [K (M) Ekafiarw Maruyanfa' Mrofanq/ Suzm/ Macaw .44 7007 au cbl BY v Y K! ATTORNEY INVENTORS United States Patent ABSTRACT OF THE DESCLOSURE A high transconductance electron gun type cathode ray tube with a low second grid voltage electron gun having multigrid electrodes successively in the advance direction of electron beams from the cathode of the electron gun. Each of the grid electrodes has a central aperture therein with the diameter of the central aperture of the second grid electrode being larger than the diameter of the central aperture of the first grid electrode and the axial length of the central aperture of the second grid electrode being greater than the diameter of the central aperture of the second grid electrode.

This invention relates to a cathode ray tube, more particularly to improvements in or relating to a cathode ray tube provided with a low second grid voltage electron gun.

There are two methods for electron beam control with video signals in a cathode ray tube, namely the so-called grid modulation and cathode modulation. In the cathode modulation, however, electron beam can be controlled with lower video signals than in the grid modulation, and hence the cathode modulation is usually employed in high-transconductance electron guns.

In electron guns designed specifically for cathode modulation purpose, there is the so-called low voltage on the second grid electrode, E electron gun such that a voltage to be impressed to the second grid electrode is lower than the usual one and it is broadly employed in cathode ray tubes for cathode modulation use.

However, since in the low E electron gun the low voltage of DC 30 v. to 150 v. is impresed to the second grid electrode, as compared with a standard E electron gun in which usual voltage of DC 300 v. to 500 v. is impressed to the second grid electrode, the electron lens focusing effect of the so-called crossover point is weak so that the diameter of the crossover and a beam spot on'the fluorescent screen increase more than those in the standard E electron gun. Furthermore, the cross-over point varies considerably with an increase in the electron beam density, especially the quality of a picture in a high-light portion is worse than that with the standard E electron gun. The conventional low E electron guns have such disadvantages.

The low E electron gun is generally formed in such a manner that the influence upon the voltage for visual extinction of focused raster due to the ultra-anode .voltage on the ultra-anode voltage penetration factor is made as small as possible and that the influence upon the voltage for visual extinction of focused raster due to the second grid voltage or the second grid voltage penetration factor becomes large. However, the decrease of the ultra-anode voltage penetration factor weakens the lens effect at the cross-over point and causes the deterioration of the quality of a picture.

Accordingly the ultra-anode voltage penetration factor is determined in the relationship between the modulation characteristics and the quality of a picture or the quality of a picture of the standard low E electron gun. Consequently the ultra-anode voltage penetration factor cannot be decreased extremely so that fully favorable modulation characteristics have not been obtained.

There have heretofore been proposed various methods for removing the foregoing disadvantages but they require very complicated structure. Many high transconductance cathode ray tube electron guns have been proposed but low E electron guns are still disadvantageous in the forementioned points.

Considering these disadvantages, almost all the electric field distribution in the vicinity of the cross-over point depends upon only the voltage impressed to the second grid electrode, so that it is very diflicult to obtain electric field distributions having satisfactory focusing action as in the standard E electron gun. It is generally considered that the dimension of a beam spot on the fluorescent depends upon that of the crossover point, and accordingly in a cathode ray tube using the low E electron gun the beam spot on the fluorescent screen tends to become large and resolution decreases inevitably. In a typical low E electron gun for obtaining high transconductance an aperture of the second grid electrode is made smaller than that of the first grid electrode or the distances between the cathode and the first grid electrode and between the first grid electrode and the second grid electrode are shortened, thereby to increase the influence upon the voltage for visual extinction of focused raster due to the voltage impressed to the second grid electrode. While it is designed to decrease the influence upon the voltage for visual extinction of focused raster due to the ultra-anode voltage impressed to the third grid electrode, but a decrease in the influence of the ultraanode voltage upon the voltage for visual extinction of focused raster greatly deteriorates the quality of a picture, and hence a moderate relationship between them is Wanted. However, no favorable modulation characteristics can be obtained more than expected.

Furthermore, when electron beams emitted from the cathode are focused at the crossover point, the crossover point moves on the axis due to space charge with an increase in the electron beam. As a result of this, it is impossible to focus beam current on the fluorescent screen with a constant beam focusing voltage, namely the socalled blooming phenomenon is caused. However, if the focusing power in the neighborhood of the crossover point is strong, the focusing action is fully carried out against the action due to space charge and the crossover point, accordingly the beam spot on the fluorescent screen can be made smaller. 7.

For this purpose, there is another method such that the beam focusing voltage for focusing a beam spot into image on the fluorescent screen is made constant by weakening the action of the pre-focusing lens to thereby prevent the crossover point from moving on the axis thereof. However the diameter of the electron beam becomes large due to the decrease of the prefocusing lens action.

Accordingly, one object of this invention is to provide a high transconductance cathode ray tube electron gun having satisfactory focusing characteristics, more particularly a cathode ray tube provided with a low E electron gun.

Another object of this invention is to provide a cathode ray tube in which a load to the cathode may be reduced and the cathode is durable.

Another object of this invention is to provide a cathode ray tube in which a strong and effective electron lens is formed.

Another object of this invention is to provide a cathode ray tube having a small crossover point of electron beam, thereby producing a picture of high resolution.

A further object of this invention is to provide a cathode ray tube in which modulation characteristics are favorable.

A still further object of this invention is to provide a cathode ray tube in which the relation between the beam current and beam focusing voltage may be maintained substantially constant.

Other objects, features and advantages of this invention will be apparent from the following description taken in conjunction with the accompanying drawings, in which:

FIGURE 1 is a schematic diagram illustrating a typical high transconductance cathode ray tube electron gun heretofore employed;

FIGURE 2 is a schematic diagram illustrating a 4- electrode portion of a high transconductance cathode ray tube electron gun according to the present invention;

FIGURES 3A and 3B are similar schematic diagrams respectively illustrating other embodiments of the present invention;

FIGURE 4A schematically illustrates an electric field distribution in the 4-electrode portion of a conventional cathode ray tube electron gun;

FIGURE 43 similary illustrates an electric field distribution diagram in the cathode ray tube according to the present invention;

FIGURE 5 is a graph showing comparative curves of secondary differential coefficients of the cathode ray tube electron gun shown in FIGURE 4; and

FIGURE 6 is also a graph of comparative curves illustrating the relationship between the beam current and the beam focusing voltage of the two electron guns shown in FIGURE 4.

FIGURE 1 is an explanatory diagram schematically illustrating a typical low voltage on the second electrode, or low E electron gun heretofore employed. 1 is a first grid electrode, which is provided with a cathode 8 and a heater 9 for heating the cathode 8 to emit electron beam therefrom. On the face of the first grid electrode 1, a central aperture 2 is formed, which confronts coaxially another central aperture 4 formed on the surface of a second grid electrode 3 opposite to the first grid electrode 1, forming a crossover point therebetween as shown by a pair of representative rays 8a and 8b. The one central aperture 2 is formed larger than the other aperture 4, in which electron gun is different in structure from usual standard E electron guns. Furthermore, an aperture 4' formed on the opposite side to the central aperture 4 is opposed to a third grid electrode 5, forming a prefocusing electron lens therebetween. The third grid electrode 5 is formed in such a manner as to accelerate an electron beam having passed through the first and second grid electrodes 1 and 3. A fourth grid electrode 6 forms a main focusing electron lens for focusing the electron beam and the electron beam is focused into image on a fluorescent screen through a fifth grid electrode 7. Thus, in the ordinary representative low E electron gun the central aperture 4 of the second grid electrode 3 is determined to be smaller than the central aperture 2 of the first grid electrode 1 so as to decrease the influence upon the voltage for visual extinction of focused raster due to the ultraanode voltage impressed to the third grid electrode 5. In one example of such a low E electron gun, ground potential is applied to the first grid electrode 1, second grid voltage of DC v. to DC 150 v. is impressed to the second grid electrode 3, ultra-anode voltage of DC 8000 v. to 16,000 v. is usually impressed to the third and the fifth grid electrodes 5 and 7 and a signal of about 0 to a peak to peak of v. is impressed to the cathode 8, whereby the electron beam is modulated. A voltage impressed to the fourth grid electrode 6 is so adjusted as to produce the optimum beam focus on the fluorescent screen. In case a cathode ray tube having such electrode arrangement as shown in FIGURE 1 is operated under the condition of the voltages mentioned above, the influence of the ultraanode voltage upon the voltage for visual extinction of focused raster usually occupies 20 to 30% in a range such that a bad influence upon resolution due to the condition of a focusing field at the crossover point may be disregarded as previously described. That is, the influence of the ultra-anode voltage upon the voltage for visual extinction of focused raster is lower than about 30 to 50% in the standard E electron gun, but 20 to 30% of it is sti l influenced.

Furthermore, because of a structure which is liable to increase the influence upon the voltage for visual extinction of focused raster due to the second grid voltage impressed to the second grid electrode 3, the first and second grid electrodes 1 and 3 are placed closely and the central aperture 4 of the second grid electrode 3 is made extremely small. Consequently, the diameter of the electron beam and the angle of divergence of the beam increase as described later and as a result, the so-called blooming increases with the advancement of the modulation characteristics.

FIGURES 2 and 3 are diagrams, each schematically illustrating a 4-electrode portion of an electron gun of a high transconductance cathode ray tube, particularly a low E electron gun according to this invention. 10 is a first grid electrode, which corresponds to the first grid electrode 1 shown in FIGURE 1, and 11 is a second grid electrode and 12 is a third grid electrode.

The axial length, namely the thickness t of the second grid electrode 11 is designed to be far greater than that (usually from 0.13 to 0.2 mm.) of the second grid electrode of any cathode ray tube electron guns heretofore employed, and its value is determined according to the thickness of respective parts and the distance therebetween but it is generally preferable to be more than 1.0 mm. Where the thickness t of the second grid electrode 11 is selected in such a manner, the diameter d of an aperture of the second grid electrode 11 is chosen to be more than the diameter d of an aperture of the first grid electrode 10 in contrast with the typical low E electron gun heretofore employed. Since the diameter d of the aperture of the second grid electrode 11 has been made large, it is possible, as shown in FIGURE 3, to provide the central aperture of the second grid electrode with a considerable length in the axial direction thereof, namely an effective length t corressponding to the thickness t shown in FIGURE 2.

The diameter d of the aperture of the first grid electrode 10 can be made far larger than that in the conventional electron gun, by which the quality of a picture is prevented from deteriorating for the reasons described later. Furthermore, when the voltage for visual extinction of focused raster is held constant a load to the cathode can be reduced by increasing the diameter d, of the aperture of the first grid electrode, which makes it possible to lengthen the life of the cathode.

The voltage to be impressed to the respective electrodes, the thickness of the electrodes and the distance between the electrodes of the electron gun of this invention are relative to one another and their values cannot be determined unconditionally, but they are given as follows. That is, the thickness of the first grid electrode of the conventional low E electron gun is required to be less than 0.1 mm., which is very diflicult to produce with the present manufacturing technique, but in the present invention satisfactory modulation characteristics can be obtained even if the axial length, namely the thickness t of the first grid electrode 10 is selected to be from 0.13 to 0.20 mm. which is the thickness of a usual first grid electrode. The diameter d of the aperture of the first grid electrode in the prior art has been selected to be less than 0.8 mm. in consideration of the quality of a picture, but in the present invention the diameter of the aperture can be selected more than 1.0 mm. Therefore, the diameter d of the aperture of the first grid electrode 10 can be made very large, so that the thickness of the first grid can be chosen over a considerably wide range and accordingly it is free from the restriction on'the manufacturing technique and the modulation characteristics can be improved. That is, the influence upon the voltage for visual extinction of focused raster due to the voltage impressed to the second grid electrode 11 or the second grid voltage penetration factor which greatly affects the modulation characteristics can be expressed in the following formula in the form of J where 1 is the distance between the confronting faces of the cathode and the first grid electrode, 1 is the distance between the confronting faces of the first and second grid electrodes, t is the thicknessof the first grid electrode and K is a constant. Where the distances and 1 between the electrodes and the voltage E impressed to the second grid electrode are held constant, the decrease in the modulation characteristics due to an increase in the thickness t of the first grid electrode can be fully compensated by selecting large the diameter d of the aperture of the first grid electrode, since K is a constant.

Furthermore, if the increase of the diameter d of the aperture of the first grid electrode 10 is made in the same unit, it is very effective, since the increaseof the diameter (1 contributes to the modulation characteristics in the tube. When the diameter d of the aperture of the second grid electrode 11 is selected to be about 1.2 to 1.5 times as large asthe diameter :1 of the aperture of the first grid electrode 10, the thickness t of the second grid electrode-or the effective length t of the aperture is variable in response to the diameterd of the aperture of the third sgrid electrode, the distance'l between the second gridelectrode-and the third grid electrode or the voltage impressed to the respective electrodes. However, it is preferableto select to 1.5 times as large as the diameter d of the-first grid electrode, 'An'example of the ratio of the diameter d ofthe aperture of the first gn'd electrode .is as follows:

Example Ratio I, mm. II, mm.

In an example of the actual operation voltage, where the first grid electrode is grounded and voltage of 50 y. is'applied to the second grid electrodell and voltage of 12 kv. is impressed respectively to the third grid electrode 12 and the fifth grid electrodecorresponding to the grid electrode 7 in FIGURE 1, the cathode voltage for visual extinction of focused raster is 30 to "40 v. and the perveance which shows the modulation characteristics can be made more than 6.0 av.-

The voltage 40 to 70 volts impressed to the second grid electrode presents favorable resolution. Furthermore, the ultra-anode voltage penetration factor can be held less than "10%.

The focusing effect near the cross-over point of the cathode ray-tube .is within-the effective length t of the second electrode 11 as indicated by the representative rays 8a, 8b. I

The foregoing will be seen from FIGURES 4 and 5. That is, FIGURE 4A schematically illustrates the electric field distribution near the 4-electrode portion of a typical low E electron gun heretofore employed, and FIGURE 4B similarly shows that of a low E electron gun according to this invention. FIGURE 5 illustrates the secondary differential coefiicient of the electric field distribution shown in FIGURE 4, in which the abscissa exseen from these curves that there is a definite difference in lens formation near the crossover point. That is, it is well-known that the strength or the focal distance of the electron lens is a function of the following formula where 11 is the secondary differential coefficient and is the potential at that poin The dotted-lined curves Hand 16 show F expressed by the above formula, that is, the curve 15 shows F in the representative conventional low E electron gun and the curve 16 illustrates F in the electron gun of the present invention. The value obtained by integrating the curve 15 or 16 in FIGURE 5 from the 0 point to the point at the abscissa corresponding to zero of the value F is the strength of the electron lens respectively. Comparing the conventional electron gun with that of this invention in this respect, it will be seen that the integrated value in the present invention is more than 30% larger, as compared with that in the conventional electron gun, namely a strong and effective electron lens is formed in this invention. Although the conventional low E electron gun is so formed as to increase the second grid voltage penetration factor and to decrease the ultra-anode voltage penetration factor, satisfactory modulation characteristics cannot be obtained in relation to the quality of a picture. Furthermore, it is necessary to weaken a pre-focusing lens so as to prevent blooming, but it cannot be effected satisfactorily in the conventionalelectron gun on condition that the modulation characteristics are liable to become worse in such a case.

That is, owing to a lens formed close to the cathode, the angle of divergence of electron beam is increased at the crossover point as illustrated by the curve 15 and it opposes. the purpose of decreasing the dimension of the beam spot. In the electron gun of the present invention, on the contrary, the crossover point is formed at a considerable distance from the cathode as shown by the curve 16 and a continuous and strong focusing lens is formed up to the pre-focusing lens, and accordingly the angle of divergence of electron beam becomes small and a small crossover point can be formed. Namely, the length of the focusing lens near the crossover point in the present invention is more than three times as long as that in the conventional electron gun.

As a result of this, the focusing effect due to the electron lens near the crossover point is increased further, by which a small crossover point can be formed and the angle of divergence of electron beam is made extremely smaller than that in the conventional electron gun by the strong focusing effect of a lens continuously formed. Therefore, the movement of the crossover point can be substantially neglected, which movement is caused by the influence of space charge when the electron beam is increased. Furthermore, even when the focusing voltage of the electron beam, namely the voltage impressed to the fourth grid electrode is held constant, a favorable beam spot can always be obtained on the fluorescent screen irrespective of the beam current.

FIGURE 6 shows the relationship between the electron beam current and the beam focusing voltage, in which the curve 17 shows the relationship between an electron beam current I and a beam focusing voltage E in cathode ray tube electron guns heretofore employed and the curve 18 shows a similar relationship in the electron gun of a cathode ray tube according to this invention.

The focusing voltage of the electron beam in the conventional electron gun varies from to 200 v. in response to variations in the beam current 100 ,ua., but in 7 a cathode ray tube provided with the low E electron gun of this invention variations in the focusing voltage can be held less than 50 v. in response to variations in the beam current of more than 500 ,ua.

The thickness t or the effective length I of the second grid electrode of this invention is related to the distance between the electrodes forming an electron lens but it is also relative to the diameter d of the aperture of the second grid electrode and further it is concerned with the diameter d;, of the aperture of the third grid electrode and to the voltage impressed thereto.

By making large the diameter d of the aperture of the second grid electrode, the aperture of a lens formed can be made large, so that spherical aberration can be removed.

Furthermore, since the electron lens formed in the present invention is stronger than that in the conventional electron gun, its focusing effect is large and spherical aberration is small. In addition, the crossover point is formed at a distance from the cathode, as compared with that in the conventional electron gun, so that the angle in which the electron beam enters into the electron lens from the cathode is considerably small. Therefore, according to the present invention, resolution of the beam spot on the fluorescent screen can be prevented from deterioration even if the diameter d of the first grid electrode is made large as compared with that of the prior art.

According to this invention, by selecting substantially equal the diameter d: of the aperture of the second grid electrode and the thickness t or the effective length t of the aperture or by selecting the thickness t or the effective length t rather greater than the diameter d the ultra-anode voltage penetration factor due to the voltage impressed to the third grid electrode is extremely decreased, while that due to the voltage impressed to the second grid electrode is increased. With a strong lens formed between the second and third grid electrodes, the angle of divergence of the electron beam at the crossover point is decreased and the blooming encountered in the prior art is suppressed, which makes it possible to improve the modulation characteristics and to reduce the diameter of the beam. As a result of this, the diameter d of the aperture of the first grid electrode can be made extremely larger than that in the conventional electron gun and the relation between the beam current and the focusing voltage thereof can be held constant.

The structures of the respective parts depend upon the voltages to be impressed to the respective electrodes as previously described, but they can be designed suitably for particular uses.

It will be apparent that many modifications and variations may be effected without departing from the scope of the novel concept of this invention.

What is claimed is:

1. A high transconductance electron gun type cathode ray tube with a low second grid voltage electron gun having a cathode, a first grid electrode, and a second grid electrode successively in the advanced direction of electron beams from the cathode of said electron gun, each of said first grid and said second grid electrode having a central aperture therein, the diameter (d of the central aperture of said second grid electrode being larger than the diameter (d of the central aperture of said first grid electrode, and the axial length (t of the central aperture of said second grid electrode being larger than the diameter (d of the central aperture of said second grid electrode.

2. A high transconductance electron gun type cathode ray tube as recited in claim 1, wherein the diameter (d of the aperture of said first grid electrode is more than 1.0

3. A high transconductance electron gun type cathode ray tube as recited in claim 1, wherein the diameter (d of the central aperture of said second grid electrode is selected to be 1.2 to 1.5 times as large as the diameter (d of the central aperture of said first grid electrode.

4. A high transconductance electron gun type cathode ray tube with a low second grid voltage electron gun having a cathode, a first grid electrode and a second grid electrode successively in the advanced direction of electron beams from the cathode of said electron gun, each of said first grid and said second grid electrode having a central aperture therein, the diameter (d of the central aperture of said second grid electrode being larger than the diameter (d of the central aperture of said first grid electrode, and the axial length (t of the central aperture of said second grid electrode being nearly equal to the diameter (d of the central aperture of said second grid electrode.

5. A high transconductance electron gun type cathode ray tube as recited in claim 4, wherein the diameter (d of the aperture of said first grid electrode is more than 1.0

6. A high transconductance electron gun type cathode ray tube as recited in claim 4, wherein the diameter (d of the central aperture of said second grid electrode is selected to be 1.2 to 1.5 times as large as the diameter (d of the central aperture of said first grid electrode.

7. A high transconductance electron gun type cathode ray tube with a low second grid voltage electron gun having a cathode, a first grid electrode and a second grid electrode successively in the advanced direction of electron beams from the cathode of said electron gun, each of said first grid and said second grid electrode having a central aperture therein, the diameter (d of the central aperture of the second grid electrode being nearly equal to the diameter (d of the central aperture of said first grid electrode, and the axial length (1 of the central aperture of said second grid electrode being nearly equal to the diameter (d of the central aperture of said second grid electrode.

8. A high transconductance electron gun type cathode ray tube with a low second grid voltage electron gun having a cathode, a first grid electrode and a second grid electrode successively in the advanced direction of elec tron beams from the cathode of said electron gun, each of said first grid and said second grid electrode having central apertures therein, the diameter (d of the central aperture of said second grid electrode being nearly equal to the diameter (d of the central aperture of said first grid electrode, and the axiallength (t of the central aperture of said second grid electrode being larger than the diameter (d of the central aperture of said second grid electrode.

References Cited I UNITED STATES PATENTS JAMES W. LAWRENCE, "Primary Examiner. V. LAFRANCHI, Assistant Examiner. 

